A solid state electrolytic capacitor is made from a porous pellet of sintered tantalum powder, a dielectric tantalum oxide layer formed on the surface of the sintered tantalum powder, a solid-state conductor impregnated into the volume of the pellet, and external connections such as silver paint, etc. The tantalum forms the positive electrode of the capacitor, and the solid-state conductor forms the negative electrode (also called the cathode or counter-electrode).
Manganese dioxide has been utilized as the cathode of choice for solid tantalum capacitors since the commercial introduction of this style of capacitor in the early 1950's. A key property of manganese dioxide is its self-healing ability. At defective portions of the dielectric film, the manganese dioxide becomes non-conductive. This is due to the manganese dioxide transforming to a lower manganese oxide because of joule heating at the defect site. This mechanism allows capacitors with low leakage currents to be produced. It also allows small dielectric defects that occur during manufacture and use to be isolated. However, if the dielectric defect is too large, the dielectric can crack. Manganese dioxide is a powerful oxidizing agent. When it comes in direct contact with tantalum through a crack in the oxide, the capacitor can ignite, leading to destruction of the capacitor and possible destruction of other components in the circuit. It is desirable to replace the manganese dioxide with a solid-state conductor that does not cause the tantalum to ignite while maintaining the self-healing ability.
The use of tantalum capacitors in high frequency circuits has become more important. This has led to the need for tantalum capacitors having low equivalent series resistance (ESR). The best manganese dioxide has a resistivity of 0.5 to 10 ohm-cm. It is desirable to replace the manganese dioxide with a solid-state conductor that has a lower resistivity. However, many highly conductive metals and oxides do not have a self-healing ability and thus are not suitable for solid-state tantalum capacitors.
Conductive polymers such as polypyrroles, polyanilines, and polythiophenes have resistivities 10 to 100 times less than that of manganese dioxide. Since they are much less powerful oxidizing agents than manganese dioxide, these materials do not cause the capacitor to ignite upon failure. Polypyrrole was shown to have a self-healing mechanism (Harada, NEC Technical Journal, 1996). Due to these favorable properties of conductive polymer compounds, these compounds are being investigated as possible replacement materials for manganese dioxide in solid-state tantalum capacitors.
Three methods have been used to deposit the conductive polymer in the porous tantalum pellet:
1. Chemical oxidative polymerization; PA1 2. Electrolytic oxidative polymerization; and PA1 3. Deposition of a polymer from solution followed by oxidation and/or doping. PA1 (a) a porous pellet anode; PA1 (b) a dielectric oxide film formed by oxidizing a surface of the porous pellet; PA1 (c) a conductive polymer counter electrode adhered to the oxide film, the conductive polymer having a silane coupling agent incorporated therein. PA1 (a) providing an oxidized porous capacitor pellet; PA1 (b) dipping the pellet in a solution comprising a solvent, a monomer, an oxidizing agent, a dopant, and a silane coupling agent; and PA1 (c) applying heat to the pellet so as to evaporate the solvent, thereby forming a conductive polymer film having the silane coupling agent incorporated therein.
In chemical oxidative polymerization, a monomer, an oxidizing agent, and a dopant are reacted inside the porous pellet to form the conductive polymer. Monomers include pyrrole, aniline, thiophene, and various derivatives of these compounds. The oxidizing agent can be either an anion or a cation. Typical anion oxidizers are persulfate, chromate, and permanganate. Typical cations are Fe(III) and Ce(IV). The best dopants are anions of strong acids such as perchlorate, toluenesulfonate, dodecylbenzenesulfonate, etc. The reaction between monomer, oxidizing agent, and dopant can take place in a solvent such as water, an alcohol, a nitrile, or an ether.
Several methods have been used to get the monomer, oxidizing agent, and dopant into the porous pellet and carry out the conversion to conductive polymer. In one method, the pellet is first dipped in a solution of the oxidizing agent and dopant, dried, and then dipped in a solution of the monomer. After the reaction is carried out, the pellet is washed and then the process is repeated until the desired amount of polymer is deposited in the pellet. In this method, it is difficult to control the morphology of the final polymer. It is also difficult to control the exact reaction stoichiometry between the monomer and the oxidizing agent. Control of this stoichiometry is critical to achieve the highest conductivity polymer (Satoh et al., Synthetic Metals, 1994). Cross contamination of the dipping solutions is a problem. Since the pellet must be dipped twice for each polymerization, the number of process steps is greatly increased. The excess reactants and the reduced form of the oxidizing agent need to be washed out of the part. This adds even more process steps and complexity to the process.
In a related method, the sequence is reversed so that the pellet is dipped in the monomer solution first and the solvent is evaporated away. The pellet is then dipped in the oxidizing agent/dopant solution and the reaction is carried out. This method suffers from all the disadvantages of the previous method. In addition, some monomer may be lost in the solvent evaporation step.
In yet another method, all components are mixed together and the pellet is dipped in the combined solution. This method reduces the number of dips and allows more precise control over the reaction stoichiometry. However, the monomer and oxidizing agent can react in the dipping bath, causing premature polymerization and loss of reactants. This adds some additional complexity and cost to the process. This is especially a problem with pyrrole monomer and Fe(III) oxidizing agents. To overcome this problem to some extent, the dipping bath can be kept at cryogenic temperature (Nishiyama et al., U.S. Pat. No. 5,455,736). However, use of cryogenic temperatures adds considerable equipment and operational complexity to the process. The pyrrole/Fe(III) can be replaced with a monomer/oxidizing agent combination that is less reactive; for example, 3,4-ethylenedioxythiophene and an Fe(III) salt of an organic acid (Jonas et al., U.S. Pat. No. 4,910,645).
In electrolytic oxidative polymerization, the monomer is oxidized to polymer at an electrode and the dopant is incorporated from the electrolyte. This polymerization method produces high conductivity polymer films. There is no chemical oxidizer to wash out of the film after polymerization.
Direct electrolytic oxidation of monomer to polymer is difficult because of the high resistance dielectric oxide layer. Various methods have been proposed to circumvent this problem. One method is to form the polymer on the tantalum and then to form the oxide layer (Saiki et al., U.S. Pat. No. 5,135,618). In another method, the polymer and the oxide layer are formed at the same time (Saiki et al., European Patent Application 0 501 805 A1). However, the electrolytes best suited for depositing conductive polymer and tantalum oxide films are quite different; therefore, these methods produce neither an optimum polymer nor an optimum oxide.
Another method is to deposit a thin film of conductive material by chemical methods, followed by contacting this layer with an electrode to carry out the electrolytic oxidative polymerization. Manganese dioxide prepared by pyrolysis of manganese nitrate (Tsuchiya et al., U.S. Pat. No. 4,943,892), manganese dioxide prepared by pyrolysis of permanganate (Kudoh et al., J. Power Sources, 1996), and conductive polymer prepared by chemical oxidative polymerization (Yamamoto et al., Electronics and Communications in Japan, 1993) have been used for this thin layer. Contacting this thin layer of conductive material with an auxiliary electrode is difficult to achieve in practice. Thus, Tsuchiya et al. propose bridging the anode lead to the conductive layer. This bridging layer must be removed after depositing the polymer by electrolytic oxidative polymerization. A complicated series of insulating washers under the bridging layer is used to accomplish this. Kojima et al. (U.S. Pat. No. 5,071,521) propose contacting the thin conductive layer with an auxiliary electrode. Use of an auxiliary electrode greatly increases process complexity, especially with sintered pellet-type anodes where an individual electrode must be provided for each individual anode. Contacting the layer with an auxiliary electrode can cause damage to the oxide layer.
In principle, direct deposition of polymer from solution involves dipping the capacitor in the polymer solution and then evaporating the solvent away to form a conductive film. This operation would be repeated several times to deposit the required amount of polymer in the pellet. This strategy would reduce the number of process steps compared to the chemical oxidative polymerization approach and would eliminate the cumbersome auxiliary electrodes used in the electrolytic oxidative polymerization approach. However, capacitance efficiency is poor with this process due to the difficulty of impregnating small pores with a liquid containing a dispersed solid phase.
Furthermore, technical limitations on conductive polymer solutions prevent this ideal process from being achieved in practice. For example, polyaniline is soluble in NMP in the emereldine base form (PANI-EB), but not in the doped form. A solution of PANI-EB is impregnated into a pellet followed by solvent evaporation to leave a low-conductivity PANI-EB film. The pellet must then be further soaked in a solution of a dopant to change the film into the conducting emereldine salt (ES) form. This doping reaction takes a considerable amount of time, and the excess dopant must be washed from the pellet. In addition, PANI-EB solutions are very viscous in concentrations above 5 wt % and tend to gel with standing. Thus, Sakata et al. (U.S. Pat. No. 5,457,862) state that PANI-EB in NMP can only be used to coat the outside of the porous pellet and is not suitable for internal impregnation. Even after doping, the resistivities of PANI-ES prepared using this method are only about 1 ohm-cm.
To avoid the gelling problem, Abe et al. (U.S. Pat. No. 5,436,796) use a solution of polyaniline in the leuco emereldine base form (PANI-LEB). This allows higher concentrations of PANI to be used without the problems of gelling, and the ultimate resistivity is lower. However, in order to be converted into the conducting PANI-ES form, the PANI-LEB films must be oxidized and doped inside the capacitor pellet. The oxidizing/dopant reaction takes a considerable amount of time, and both the excess dopant and excess oxidizer mush be washed from the pellet.
Despite the considerable research and development efforts to capitalize on the intrinsic advantages of conductive polymer electrodes, devices manufactured with these materials have been met with only limited commercial success. The application of stable, continuous, highly conductive, adherent films to interior and exterior surfaces of porous anodes presents numerous technical challenges. One of the difficulties associated with conductive polymer solid electrolytes is poor adherence between the dielectric oxide and the conductive polymer, particularly following exposure to high humidity environments. As a consequence of the poor adhesion between dielectric oxide and solid polymer electrolyte, capacitance efficiency is reduced. Capacitance efficiency is defined as the ratio of the dry capacitance of the capacitor following application of the solid electrolyte to the capacitance of the capacitor as measured in a suitable wet electrolyte prior to the application of the solid electrolyte coating. Capacitance efficiencies less than 1.0 increase the cost of manufacturing tantalum capacitors, since additional tantalum is required to obtain the desired capacitance. The continuing market trend toward miniaturization of electronic components places a premium on capacitance efficiency since a larger anode is required to compensate for capacitance loss.
Poor adhesion of the polymer film to the external surfaces of the porous tantalum anode result in cracking and peeling, creating a discontinuous exterior polymer film. Application of silver paint to a cracked exterior film leads to a high leakage current. Poor adhesion of the polymer film to the tantalum oxide dielectric surfaces can also cause an increase in dissipation factor of the capacitor. A method to improve adhesion of the conductive polymer film to the dielectric surfaces is thus highly desirable.
The use of organosilanes to serve as coupling agents between oxides such as those of aluminum, zirconium, titanium, tin, and nickel and organic phases is reported by Arkles ("Tailoring Surfaces with Silanes," Chemtech 7, 766, 1977). Wu et al., writing in Chemistry of Materials, Volume 9, Number 2, February 1977, reported improved adhesion between polyaniline and glass slides. The glass surface was modified through application of an amino silane prior to deposition of polyaniline on the glass substrate. Sato et al. claim improved adhesion of polypyrrole to anodized aluminum films in Japanese Patent Number 09246106. Sato et al. disclose the use of silane coupling agents as a pretreatment, but do not teach the incorporation of silanes into the polymerization solution.
Sakata et al. (U.S. Pat. No. 5,729,428) describe a solid electrolytic capacitor having a valve action metal body, an oxide film as a dielectric from oxidizing the metal body, an electron donor organic layer formed from an organic compound having an electron donor group, and a conductive polymer layer as a solid electrolyte layer covering the entire surface of the electron donor organic compound. Sakata et al. also teach two ways of making the electron donor organic layer in the capacitor which uses conductive polymers as solid electrolytes, and a method for making the capacitor.
The electron donor organic layer taught by Sakata et al. may be comprised of fatty acids, aromatic carboxylic acids, anionic surface active agents, phenol and its derivatives, hydrolysates of silane coupling agents, titanium coupling agents, or aluminum coupling agents which covers the entire surface of the oxide film with a thickness of a monolayer to several layers. Sakata et al. describe suitable silanes which include 3-glycidoxypropyltrimethoxysilane.
Sakata et al. disclose forming the electron donor organic layer by either contacting the oxide with the vapor of the electron donor organic compound or dipping the oxidized anode in an alcohol solution of the electron donor compound (specifically a 2 wt % solution of 3-glycidoxypropyltrimethoxysilane in methanol). The reference also teaches that the water in aqueous solutions tends to react with the oxide, thus preventing the coupling agent's reaction (although the silane agents will allow formation of a thin film when contained in an acidic aqueous solution).
Sakata et al. disclose that the purpose of the electron donor organic is to increase adhesion between the oxide and conductive polymer, thereby preventing reduction of electrostatic capacitance and preventing deterioration in dissipating factor at high temperatures. Sakata et al. also warn to keep the electron donor organic layer thickness minimal as increased thickness leads to increased equivalent series resistance and reduced electrostatic capacitance, but do not teach incorporating the silane into the polymerization solution.